Tin-plating process



Patented Dec. 16, 1947 UNITED STATES PATENT orFlcE TIN -PLATING PROCESS No Drawing. Application November 13, 1943, Serial No. 510,207

1 Claim. 1

This invention relates to electrodeposition of tin and, more particularly, to electrotinning from alkali metal stannate solutions.

Alkali metal stannate solutions have long been used for the electrodeposition of tin. For many purposes these solutions have been quite satisfactory. However, the conventional stannate tin plating solutions are not efficient at high cathode current densities, for example, current densities higher than 50 amps. per sq. ft. In certain modern electrotinning operations, for example, in the continuous electrotinning of steel sheet, it is desired to use cathode current densities as high as possible, e. g. 100 amps. per sq. ft. and higher, in order to rapidly produce large quantities of the electrotinned product.

It has been found that potassium stannate solutions permit the use of much higher cathode current densities than are possible with the sodium stannate solutions. However, difiiculty has been experienced in operating potassium stannate solutions, in that the anode current efiiciency is relatively low, resulting in an unbalanced condition and preventing the use of sumciently high anode current densities.

An object of the present invention is an improved process for electrotinning from alkali metal stannate solutions. A further object is to improve the process of electrotinning from a potassium stannate bath. Another object is to provide means for increasing the anode current efficiency in electroplating tin from a solution containing potassium stannate. Still other objects will be apparent from the following description of the invention.

It has now been found that the anode current efiiciency in a potassium stannate plating bath can be markedly increased by the addition of relatively large amounts of potassium carbonate. In accordance with the present invention, sufficient carbonate -is added to a potassium stannate plating bath so that the carbonate concentration is equal to 15 to 30 ounces per gallon of potassium carbonate. By way of example, potassium stannate baths suitable for practicing the present invention may contain to 25 ounces per gallon of potassium stannate, 2 to 6 ounces per gallon of potassium hydroxide and to 30 ounces per gallon of potassium carbonate.

t is known that during the electrolysis of alkali metal stannate solutions, more or less carbonate tends to form in the'bath due to the absorption of carbon dioxide from the air. However heretofore excessive formation of carbonate in a tin plating bath has been considered deleterious and it has proved to be deleterious in sodium stannate plating solutions. In commercial operations the amount of carbonate thus formed by carbon dioxide absorption seldom exceeds 10 to 12 ounces per gallon and the preferred practice has been to remove the carbonate or to discard the bath when such high concentrations of carbonate were reached.

While such precautions are necessary for efficient operation of a sodium stannate plating bath, it has now been discovered that high concentrations of carbonate are not deleterious in potas sium stannate plating baths and that unusually high concentrations of 15 to 30 ounces per gallon of potassium carbonate are advantageous in potassium stannate plating baths in increasing the anode current efficiency without appreciably efiecting cathode current efiiciency.

This efiect is quite different from that produced by a high concentration of carbonate in a sodium stannate plating bath. In a sodium stannate bath the addition of large amounts of carbonate decreases the cathode efliciency and tends to slightly decrease the anode current efficiency.

It further has been discovered that the ad dition of the aforesaid high concentration of carbonate to a stannate plating bath containing both potassium and sodium compounds has substantially the same efiiciency-increasing effect at the anode as in the bath composed solely of potassium compounds, provided that the proportion of sodium compounds in the bath is not too high. It has been found that this effect is produced so long as the molar ratio of total sodium to potassium ions in the bath is not greater than 1:1; preferably it should, not be greater than 0.8 to 1.

In using mixed baths containing both potassium and sodium compounds in accordance with this invention, the sodium and potassium compounds may be supplied as stannate hydroxide or carbonate, as desired. For example, sodium carbonate or sodium hydroxide, or both, may be added to a potassium stannate plating solution in suitable proportions. Or, sodium and potassium stannate solutions may be mixed and suitable amounts of potassium or sodium hydroxide and carbonate added. All the various possible Ways of preparing such mixed potassium-sodium stannate plating baths in accordance with the present invention will be apparent to the skilled electroplater.

An example of a suitable mixed stannatepiat ing solution is:

. Grams per liter Sodium stannate, Na2Sn(OH)6 120 Sodium hydroxide, NaOH 20 Potassium carbonate, K2003 120 Molar ratio Na+ to K =0.8 to 1 V The above-described efiects of carbonate concentration have been demonstrated by an experi ment in which six electroplating baths having. the composition described below were connected in series and simultaneously operated with pure tin anodes and sheet steel cathode in each bath. Each bath was operated at an anode current density of 55 amps. per sq. ft. and a cathode current density of 55 amps. per sq. ft. and the temperature of each bath was maintained at 90 C. The composition of the six baths was as follows:

his used, it introduces sodium ions in the bath and, if necessary, sufficient potassium compound or With this combination, anode current densities .in the range 100 to 200 amps. per sq. ft. can be used and thus it is relatively easy to maintain a balanced bath while plating at exceedingly high speeds with cathode current densities in excess-0f 200 amps. per sq. ft. Such conditions are especially desirable for electrotinning steel sheet in a continuous process.

While it is preferred to operate with either substantially pur tin or tin-alkali metal alloy Bath Ingredients in Grams Per Liter Solution No.

The anode and cathode current efficiencies for each of the six baths were determined with the following results.

1 Best bath.

Although the electroplating solutions of the present invention may be operated at from room temperature or at elevated temperatures up to the boiling point, it is preferred to maintain the bath temperature at 80' to 95 C. At such temperatures the anode may be operated at a current density of 60 to 80 amps. per sq. ft. while the cathode may be operated at 200 amps. per sq. ft. or higher.

For high-speed plating it is generally desirable to operate the anode at a current density even higher than 80 amps. per sq. ft. This may be done in accordance with the present invention by using in combination the high carbonate concentration and at the same time using a tinalkali metal alloy anode such as that described in ccpending application by Newell F. Blackburn, Serial No. 487,813, filed May 20, 1943, now Patent 2,406,189. Such alloy anodes are composed of tin and 0.1 to 10% by weight of sodium or potassium, or both. In practicing the present invention it is generally preferred to use a tin alloy anode containing about 0.1 to 5% by weight of sodium or potassium. When a sodium-tin anode anodes, the invention is not restricted thereto. Any tin-bearing anode containing not less than about by weight of tin is suitable for practicing the herein-described invention.

I claim:

A tin plating process which comprises electrolyzing with anodes consisting of an alloy of tin and sodium containing about 0.1 to 10% by weight of sodium in a Water solution consisting of 120 grams per litre of sodium stannate, 21 grams per litre of potassium hydroxide and 120 grams per litre of potassium carbonate, at a temperature of 90 C., an anode current density within the range to 200 amperes per square foot and a cathode current density of not less than 200 amperes per square foot.

NEWELL F. BLACKBURN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,841,978 Oplinger Jan. 19, 19321 2,406,189 Blackburn Aug. 20, 1940 FOREIGN PATENTS Number Country Date 299,794 Germany July 26, 1922 471,281 Germany Feb. 9, 1929 OTHER REFERENCES Transactions Electrochemical Society, vol. 82, pp. 77-100 (1942).

Preprint 84-13, Electrochemical Society, p. 121 (1943). 

